This invention relates to a process and an apparatus for obtaining a high-hydrogen, low-carbon-monoxide gas (particularly for feeding low-temperature fuel cells in electric vehicles) by means of a methanol reforming reaction, using a Cu- and ZrO.sub.2 -containing catalyst material. The process and apparatus according to the invention produces a high-hydrogen gas mixture whose CO content is reduced to less than 100 ppm.
A process and apparatus of this type are disclosed in U.S. Pat. No. 4,840,783. There, the Cu constituent is provided in the reforming reactor, for example, by introducing CuO into the reactor and reducing it to form elemental copper. This is accomplished by causing an N.sub.2 /H.sub.2 mixture which has an H.sub.2 content of 3% to pass through at temperatures of between 100.degree. C. and 270.degree. C. for 72 h. In a subsequent stage, the initial gas mixture of the methanol reforming reactor is subjected to a selective methanization for reducing the CO concentration.
A detailed process for selective methanization is disclosed in U.S. Pat. No. 3,616,164, in which a ruthenium or rhodium catalyst on an aluminum oxide carrier material is used. By means of this process, the CO concentration in the gas mixture can be lowered to less than 100 ppm. It is also known that, in addition to the selective methanization, the CO content in a high-hydrogen gas mixture can alternatively be decreased by the selective oxidation with the use of a platinum, rhodium or ruthenium catalyst material on an Al.sub.2 O.sub.3 carrier (e.g., U.S. Pat. No. 3,631,073), or by the selective hydrogen separation by means of a membrane consisting of a palladium alloy (e.g., Japanese Published Patent Application JP 62-105901(A)).
In Chemical Abstracts, Volume 115, 1991, No. 240892d, a ruthenium/titanium catalyst is disclosed for the selective methanization of carbon dioxide at room temperature and atmospheric pressure.
Chemical Abstracts, Volume 74, 1971, No. 35106u, describes the selective methanization of carbon monoxide in a gas mixture, which contains hydrogen, carbon dioxide and carbon monoxide, at temperatures in the range of between 125.degree. C. and 300.degree. C. using ruthenium-containing catalysts.
Published European Patent Applications EP. 0 650 922 A1 and EP 0 650 923 A1 describe processes and apparatuses for the catalytic removal of carbon monoxide in a high-hydrogen gas by selective oxidation and/or selective methanization. For the selective oxidation, Pt/Al.sub.2 O.sub.3, Ru/Al.sub.2 O.sub.3 or Pt zeolite carrier catalyst material are suggested. For the selective methanization, the use of an Ru/Al.sub.2 O.sub.3 carrier catalyst material is suggested.
One object of the present invention is to provide a process and an apparatus of the initially mentioned type which produces a high-hydrogen and low-carbon-monoxide gas suitable for feeding low-temperature fuel cells, in a mobile systems, with expenditures which are as low as possible.
This object is achieved according to the invention by means of a methanol reforming reaction using a Cu- and ZrO.sub.2 -containing material, followed by removal of carbon monoxide by selective methanization on a special ruthenium catalyst material with a TiO.sub.2 carrier and/or by means of a selective oxidation using a platinum catalyst material with a TiO.sub.2 carrier. In this manner, a very high methanol conversion rate of over 98% can be achieved in the case of a high integral catalyst load of up to 10 Nm.sup.3 H.sub.2 /kg.sub.cat h, as well as a CO concentration of less than 100 ppm, preferably below 50 ppm. (Nm.sup.3 refers to a cubic meter, at standard conditions.)
In a further embodiment of the invention, the Cu constituent of the catalyst is provided by reducing CuO by means of an N.sub.2 /H.sub.2 mixture with an H.sub.2 content of more than 5% and up to 20%. In this manner, a sufficiently active catalyst material is obtained which withstands the above-mentioned, high integral catalyst load, to which, in addition, an isothermal reaction course may contribute which has a high gas flow rate.
According to another feature of the invention, it is found that, a particular composition of the TiO.sub.2 -containing catalyst carrier material is favorable for selective oxidation of the carbon monoxide.
Another advantage for the selective methanization and/or the selective oxidation is an unsteady reaction course which has a periodically changing flow direction at temperature below 200.degree. C. and 120.degree. C. Thus, both types of the selective CO conversion can be used together.
A further embodiment of the invention contains a methanol reforming reactor in a tube bundle construction which is designed specifically for a high integral catalyst load with a high methanol conversion rate.
To improve the heat transfer capacity of the tube bundle arrangement of the methanol reforming reactor, in still another embodiment, the reaction tube or tubes are provided with heat transfer ribs arranged on their side facing the tempering fluid, and/or on their side facing the catalyst material, with heat conduction webs.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.